1. Field of the Invention
The invention relates to electric lamps and particularly to high intensity discharge lamps. More particularly the invention is concerned with reduced mercury, miniature high intensity discharge lamp.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
The use of low wattage, high intensity discharge lamps for automotive forward lighting is well established. All of the major lighting companies have products on the market. These types of lamps have been used in the automotive market for a number of years and offer more light on the road for improved nighttime visibility. Most of these lamps contain mercury in small amounts, although there has been considerable effort to develop mercury-free versions.
In most cases, miniature HID lamps perform well over the expected life of about 2500 hours. However, because of the small volume and high power loading, the interior of the lamp vessel is quite hot and physiochemical reactions that affect the virgin lamp performance are accelerated. As a result, the color of the lamp may shift over time. Regulations require the color initially to be within the SAE or ECE defined white areas; nonetheless the chromaticity coordinates may shift over the lamp's life due to chemical reactions within the lamp volume. Color shift is a major concern of vehicle manufacturers and their customers. Customers prefer a minimal color shift; a slow color shift and a color shift such that neither the x nor the y color coordinates go below 0.360 within the first 1500 hours of lamp operation. Of course, the lamp arc gap and operating voltage must be the same for optical considerations and the lamp must function properly on existing electronic control gear. There is then a need for a miniature automotive HID lamp with increased color stability, voltage stability, and lumen maintenance. There is a similar need for such a lamp with increased lamp life. The present invention addresses the color shift of the light output from the automotive headlamp as the lamp ages.